How to manipulate position of mask layer using python
I don't think you can get the position of the mask as a whole, but you can access the coordinates of each individual point of the mask.

You can access the mask with: 

bpy.data.masks['Mask']


And then the layer within the mask:

bpy.data.masks['Mask'].layers['MaskLayer']


Then you have access to the splines in that layer, which are the curves of the mask. In this case I am accessing the first spline:

bpy.data.masks['Mask'].layers['MaskLayer'].splines[0]


Which then leads you to the spline's points. The points have a vector called co (coordinates) which store the x and y position of that point. By looping over all the points in the mask and altering the co[0](x) or co[1](y) you can move the mask:

mask = bpy.data.masks['Mask'].layers['MaskLayer'].splines[0]
for point in mask.points:
    point.co[0] += 0.1